Optical fiber has, for several years, been used in telecommunication networks as “backbones” to link central offices (COs) to one another. In contrast, however, ordinary twisted pairs of copper wire (“wire-pairs”) have continued to be used as “local loops” to link the COs to customer premises.
Telecommunication companies have now begun the process of transitioning to optical fiber all the way out to customer premises so they can offer the same broadband access as do cable companies. However, unlike wire-pairs, optical fibers by their nature require each customer premises to have terminating equipment (“customer premises equipment” or CPE) so their optical signals can be converted to electrical signals.
Unfortunately, optical fibers are unsuited to convey the electric power the CPE requires to operate. The CPE must derive its electric power from a separate source. Of course, the CPE could be plugged into the commercial power grid at the customer premises. This is a satisfactory solution until the power grid fails. The CPE loses its source of power, and telecommunication services are interrupted until power is restored. Given the historical reliability of the nation's telecommunication system, this is unacceptable.
One approach to ensuring that power to CPE is maintained at the customer premises during power grid failures is to place an uninterruptible power supply (“UPS”) at the customer premises. Unfortunately, UPSs are battery-powered and must be monitored and maintained to ensure that they operate when needed.
The other approach is to provide power from the CO to each customer premises using the wire-pairs that the optical fibers superseded. Thus, the wire-pairs become conduits for electric power rather than for telephone conversations. The wire-pairs can provide power all the time or only during grid failures.
Although the latter approach avoids the need for UPSs, using wire-pairs for electric power brings its own complications. The second method historically has used bulk +/−130 to +/−190 volt (V) converters at the CO and bulk converters converting down to a voltage suitable for CPE at the customer premises. These relatively high voltages are necessary given the electrical resistance of the wire-pairs. However, to maintain safety, all wire-pairs used for local loops fall under Class 2 of the National Electrical Code (NEC), which establishes a 100 volt-ampere (VA) limit on the wire-pairs. Therefore, line-dedicated 130/190V current limiters must also be used at the CO to ensure that the 100 VA limit per twisted pair is not exceeded.
This poses several problems. First, the CO must create dedicated equipment rack space to house bulk 130V and/or 190V equipment. Rack space is in high demand. Second, bulk 130/190V power is more dangerous than the 48V power traditionally handled in the rack space and thus places an additional safety burden on both equipment design and field operation procedures. Third, current limiter behavior at the 100 VA limit is problematic at the CO, because limiting transistors contained in the current limiter may operate in their linear region, dissipating power rather than delivering it to the twisted pair. Fourth, the 100 VA limit at the customer premises is also problematic in that the CPE must always draw power below the 100 VA limit but also take into account resistive losses in the twisted pair. The bulk converters at the customer premises must therefore have current limited inputs. Finally, as power needs change from application to application, bulk converters or application-specific converters do not scale well.
Furthermore, the performance (transmission characteristics) of wire-pairs tends to change over time. Wire-pairs that are decades old tend to exhibit more resistive loss than newly installed wire-pairs. Prior art methods that employing bulk converters at the CO do not measure, and therefore compensate, for performance changes over time.
Accordingly, what is needed in the art is a system and method for providing electric power to telephone wire-pairs that meets one or more of the following objectives: does not require dedicated space, is safer than 130/190V power, does not waste power near the current limit or takes into account any changes in performance that may occur with respect to wire-pairs. What is further needed in the art is a system and method for receiving electric power from telephone wire-pairs that efficiently tailors itself to the power carrying capacity of the twisted pair. What is still further needed in the art is a system and method for providing and receiving electric power through telephone wire-pairs that scales well as application needs change.